1. Field of the Invention
The present invention relates to a Multiple Input Multiple Output (MIMO) communication system. More particularly, the present invention relates to a MIMO communication system for selecting an operation mode having a maximum throughput by using a standard deviation and an average of an instantaneous Carrier to Interference and Noise Ratio (CINR).
2. Description of the Related Art
Currently, as demand for wireless communication technologies for providing various multimedia communications is increasing, research and development in high-speed data transmission technologies is actively being pursued. A 3rd Generation (3G) wireless communication system employs a Code Division Multiple Access (CDMA) transmission scheme which has a limit in the processing of large-sized radio data. Therefore, a multi-carrier based Orthogonal Frequency Division Multiplexing Access (OFDMA) transmission scheme is considered as a transmission scheme for a next generation wireless communication system.
According to a basic concept of the OFDMA transmission scheme, serial input data streams are converted into N parallel data streams and then are carried and transmitted onto independent subcarriers in order to increase a data rate. In the OFDMA transmission scheme, the subcarriers have orthogonality and thus the respective subcarriers may overlap on a spectrum. In addition, a receiver can separate the subcarriers by using a simple signal processing scheme. As a result, an interval between transmitted symbols is longer than a case of sequentially transmitting data by using one carrier. Therefore, influence of a delay time and impulse noise of a channel is reduced. Further, interference between consecutive symbols can be reduced, thereby increasing resistance to a multi-path channel. Furthermore, a complexity of channel equalization may be reduced, and spectral efficiency may be improved in comparison with a conventional frequency division scheme.
Increasing of a transmission bandwidth is not sufficient to satisfy a data transfer rate desired by a next generation wireless communication system. There may be several methods for increasing the data transfer rate in an assigned bandwidth. At present, a method using multiple antennas is known as a most effective method.
A Multiple Input Multiple Output (MIMO) scheme may increase a transfer rate by using the multiple antennas. However, for certain reasons, it is difficult to apply the MIMO scheme to broadband transmission. To apply the MIMO scheme to broadband communication, characteristics of the OFDMA transmission scheme may be used. Therefore, active research on combining the MIMO scheme and the OFDMA transmission scheme is being conducted at present, and a new method is being proposed.
The MIMO scheme may be used to improve reception reliability for an assigned data rate or to increase a data rate for an assigned reliability. In other words, in a poor channel condition, a diversity gain can be obtained by transmitting the same data through multiple paths. In a good channel condition, the diversity gain can be obtained by transmitting individual data streams in parallel through different spatial channels.
Most research activities currently conducted in association with the MIMO scheme focus on a transmission method of obtaining a maximum diversity gain or a maximum spatial multiplexing gain. A method for switching between a diversity mode and a multiplexing mode according to a channel change is also proposed.
A link adaptive algorithm, which is a transmission mode switching technique, is designed to maximize spectral efficiency by switching between a Transmission Diversity (TD) mode and a Spatial Multiplexing (SM) mode based on a Signal to Noise Ratio (SNR) and a time/space indicator.
In the transmission mode switching technique, a MIMO operation mode (i.e., MIMO mode) is selected after a relative condition number and an average Carrier to Interference and Noise Ratio (CINR) measured by a receiver (i.e., a mobile terminal) are fed back to a base station so as to be compared with a Look-Up-Table (LUT).
However, when the average CINR is not correctly measured by the mobile terminal in a method of selecting the operation mode by using the average CINR measured by the mobile terminal, there is a problem in that the operation mode cannot be correctly selected.
Therefore, a need exists for an apparatus and method for selecting an operation mode having a maximum throughput by using a standard deviation and an average of an instantaneous Carrier to Interference and Noise Ratio (CINR).